Really revert bench trigger changes.

BUG=skia:
NOTRY=true
R=kelvinly@google.com
TBR=kelvinly@google.com

Author: bensong@google.com

Review URL: https://codereview.chromium.org/324403009
diff --git a/bench/gen_bench_expectations.py b/bench/gen_bench_expectations.py
index a6f9605..6d44b6c 100644
--- a/bench/gen_bench_expectations.py
+++ b/bench/gen_bench_expectations.py
@@ -49,6 +49,7 @@
   avg = sum(benches)/len(benches)
   minimum = min(benches)
   maximum = max(benches)
+  diff = maximum - minimum
 
   return [minimum - diff*RANGE_RATIO_LOWER - avg*ERR_RATIO - ERR_LB,
           maximum + diff*RANGE_RATIO_UPPER + avg*ERR_RATIO + ERR_UB]